The present invention relates to a method of activating a zirconia oxygen sensor which detects the oxygen concentration of an ambient atmosphere by means of a zirconia element that has a porous electrode formed on both sides of an oxygen ion conductive solid zirconia electrolyte.
The sensing element used in zirconia oxygen sensors is produced through a process that comprises the steps of forming a plate of sensor body from partially stabilized zirconia, fully stabilized zirconia, etc., attaching a platinum or other metal electrode to both sides of the plate, and firing the assembly in a temperature-controlled furnace. The thus produced sensing element, as it emerges from the firing furnace, is not suitable for immediate use since its internal resistance has increased markedly due to the oxygen which binds to the electrodes. One of the conventional methods for solving this problem is taught in Japanese Patent Unexamined Publication No. Hei 3-156361 and it consists of causing an alternating current to flow between the electrodes of the fired sensing element, so that it is activated to have a lower internal resistance.
However, in order to reduce the internal resistance of the sensing element by this conventional approach of activation, the surface of the metal grains which form the electrodes must be rendered to have a fine-grained structure by supplying the zirconia oxygen sensor with an alternating current 1 to 5 times as large as the critical current. Although this method is capable of reducing the internal resistance of the sensing element, an undesired phenomenon called "blackening" takes place in the zirconia during the activation treatment, thereby deteriorating the endurance of the sensing element.
This problem is further discussed below. First, when an alternating current 1 to 5 times as large as the critical current is admitted to activate the sensing element as in the prior art, the concentration of oxygen at the zirconia-electrode interface becomes so low during the activation that the reaction ZrO.sub.2 .fwdarw.Zr+O.sub.2 will occur on the surface of zirconia and the structure of the affected surface area is destroyed (occurrence of cracks) to become brittle. Thus, the surface of metal grains constituting the electrode is also finely grained. Since the brittle structure is black in color, this phenomenon is generally called "blackening".
When blackening occurs during the activation treatment, if it can be recognized after the treatment, it will not be a problem since one can check for its occurrence in subsequent steps such as inspection. In fact, however, in the conventional practice of activation treatment, in addition to the application of an alternating current, depending on the other conditions for performing the treatment such as a comparatively small treating current and a comparatively high treating temperature, the sign of blackening that once occurred during the activation treatment may disappear and cannot be effectively detected after the treatment; in this case, the defective sensing element will be used as a normal sensing element.
However, zirconia which once experienced blackening is deteriorated in its internal structure, so if one attempts to reactivate the sensor by performing periodic activation treatments during its service, the internal deterioration will progress and its endurance drops to shorten its useful life considerably.
Another reason for the failure to recognize the previous occurrence of blackening after the activation treatment may be explained as follows: not only just after the activation treatment but also during subsequent use, the oxygen in the ambient atmosphere will initiate the reaction Zr+O.sub.2 .fwdarw.ZrO.sub.2 on the surface of zirconia which experienced blackening and, as a result, zirconia's surface which once turned black and brittle reverts to the initial white color.